Low back pain (LBP) is the #1 cause of disability worldwide and a leading cause among Veterans. LBP has a recurrent course for most individuals, involving the periodic occurrence of new episodes that patients must manage. New LBP episodes therefore are critical juncture points where interventions hold promise for optimizing functional recovery, promoting return to work (RTW), and averting a cascade of downstream consequences such as opioid use, mental health problems, and even the risk of suicide and death. The general advice to remain active during a new LBP episode is a key part of clinical practice guidelines. However, existing guidelines provide no specific recommendations about the types of activity (e.g., lifting, bending, etc.) that should be engaged in or avoided (?Which??) or the duration/intensity of such activities should they be performed (?How??). The guidelines therefore do not address a major concern of patients with LBP, which is that certain activities may have short-term effects on pain exacerbations or ?flares?, or cause sustained detrimental effects on pain or function. The distinctions between healthy, benign, and detrimental activities are also important to clinicians, who are often called upon to complete ?work restriction forms? as part of the RTW process. An ideal set of work restrictions after a new LBP episode would limit only those activities that are likely to cause flares of pain (short-term effects) or poor overall functional recovery in the longer term (overall effects). However, there exist no empirically derived data concerning the short-term vs. overall effects of specific activities during a new episode of LBP, with which to guide work restrictions. Instead, in standard practice, clinicians? recommendations to engage in or avoid specific activities rely on their personal practice style and may limit functional recovery if overly restrictive. The proposed research will use a novel approach to distinguish the short-term effects of activities from the overall effects of such activities, by conducting a longitudinal case-crossover study nested within a cohort study. This design accounts for measured and unmeasured confounds by using each case as his/her own control, analogous to a crossover experiment, capitalizing on modern mobile health and actigraphy technology. Aim 1: Estimate the short-term effects (?24 hours) of specific physical activities on pain flares in Veterans with LBP. A longitudinal case-crossover study will be conducted to identify activities that trigger flares among 550 Veterans of working age (18-65 yrs) seen for a new LBP episode in VA primary care. The primary outcome is a validated flare definition characterized by pain and functional impact. Analyses will use a biopsychosocial framework accounting for potential confounders and moderators, such as pain characteristics, work-related factors, catastrophizing, fear of movement, depression, post-traumatic stress, and opioid use. Aim 2: Estimate the overall effects of specific physical activities among Veterans with LBP. A cohort study approach to the Aim 1 study sample will examine associations between the frequency of specific activities over the first 8 weeks of follow-up, and overall LBP-related functional recovery as defined by the Roland-Morris Disability Questionnaire at 12-month follow-up (primary outcome). Analyses will also use a biopsychosocial framework accounting for a broad range of other factors, as done for Aim 1. Secondary outcomes include average pain intensity, lost work productivity, quality of life, and opioid use at 12 months. These study aims will produce the first evidence-based estimates of the short-term and overall risks of activities during LBP. These data will (1) support Veterans with LBP in navigating the complex process of recovery, (2) assist VA clinicians in providing work restrictions, and (3) inform the development of a future educational intervention for LBP that can be tested in a future RCT. Thus, this work responds directly to the Rehabilitation Research and Development service?s call for activity-based interventions for chronic pain.